Field of the Invention
The present invention relates to a magnetic recording medium. In particular, the present invention relates to a magnetic recording medium used in a hard disc magnetic recording device (HDD).
Description of the Related Art
Perpendicular magnetic recording system is adopted as a technique for achieving high-density magnetic recording. A perpendicular magnetic recording medium at least comprises a non-magnetic substrate and a magnetic recording layer formed of a hard-magnetic material. Optionally, the perpendicular magnetic recording medium may further comprise: a soft-magnetic under layer which is formed from soft-magnetic material and plays a role in concentrating the magnetic flux generated by a magnetic head onto the magnetic recording layer; an interlayer for orienting the hard-magnetic material in the magnetic recording layer in an intended direction; a protective film for protecting the surface of the magnetic recording layer; and the like.
The magnetic recording layer of the perpendicular magnetic recording medium formed of a granular magnetic material has been proposed, for the purpose of obtaining favorite magnetic properties. The granular magnetic material comprises magnetic crystal grains and a non-magnetic body segregated to surround the magnetic crystal grains. Respective magnetic crystal grains in the granular magnetic material are magnetically separated from each other with the non-magnetic body.
For the purpose of further increasing the recording density of the perpendicular magnetic recording medium, a need for reduction in the grain diameter of the magnetic crystal grains in the magnetic layer arises in recent years. On the other hand, the reduction in the grain diameter of the magnetic crystal grains leads to a decrease in thermal stability of the recorded magnetization (signals). Thus, the magnetic crystal grains in the magnetic layer need to be formed of a material with higher magnetocrystalline anisotropy, in order to compensate the decrease in thermal stability due to the reduction in the grain diameter of the magnetic crystal grains. As the material having the required higher magnetocrystalline anisotropy, L10 ordered alloys have been proposed. Typical L10 ordered alloys include FePt, CoPt, FePd, CoPd, and the like.
It has been tried to improve the magnetic properties of the magnetic recording layer comprising the L10 ordered alloy by stacking a layer of the L10 ordered alloy and a non-magnetic metallic layer. For example, Japanese Patent Laid-Open No. 2003-036514 proposes a magnetic recording medium capable to achieve easy compositional control and excellent magnetic properties by forming a magnetic recording layer, in which FePt layers and Ag layers are alternately stacked, onto a substrate consisting of spinel or the like. This proposal describes that physical separation of the FePt magnetic crystal grains can be readily achieved by using Ag having a lattice parameters comparable with those of FePt and exhibiting low solubility in Fe and Pt. Further, this proposal discloses usefulness of the spinel as the substrate, parallelly to MgO. However, it is not corroborated whether the above-described effect is obtainable in the case where the spinel is used.
It has been tried to improve the magnetic properties of the magnetic recording layer comprising the L10 ordered alloy by a layer formed under the magnetic recording layer. For example, Japanese Patent Laid-Open No. 2011-165232 proposes a magnetic recording medium in which the magnetic recording layer is formed on an interlayer comprising MgO as a principal component and one or more of additional oxides. In this proposal, facilitation of separation of the magnetic crystal grains, reduction of exchange coupling, and reduction of dispersion in coercive force are allowed by reducing the crystal grain diameter in the interlayer by addition of the additional oxides, and forming one magnetic crystal grain in the magnetic recording layer onto one crystal grain in the interlayer (hereinafter, referred to as “one-to-one formation”).
Besides, Japanese Patent Laid-Open No. 2011-146089 proposes a magnetic recording medium in which a seed layer consisting of amorphous ceramics such as SiO2, an orientation controlling layer consisting of a crystalline material such as MgO and MgAl2O4, and a magnetic layer comprising FePt alloy as a principal component are stacked in this order. In this proposal, improvement in the crystalline orientation and miniaturization of the structure of the orientation controlling layer are allowed by disposing the seed layer consisting of the amorphous ceramics. Then, the magnetic layer having a large magnetic anisotropy constant Ku, excellent magnetic properties and excellent read/write characteristics can be obtained by suppressing disturbance in crystalline orientation of the FePt alloy in the magnetic layer and reducing the grain diameter of the magnetic crystal grains.
Further, it has been tried to improve the magnetic recording characteristics of a magnetic recording layer consisting of cobalt ferrite represented by a formula of CoxFe3−xO4 (x is less than 3) by a layer formed under the magnetic recording layer. For example, Japanese Patent Laid-Open No. 2015-041392 proposes a magnetic recording medium in which a seed layer consisting of a spinel type oxide is formed between an interlayer consisting of MgO and a magnetic recording layer of the cobalt ferrite. This proposal discloses Mg1+yTi2−yO4 (y is less than 1) as an example of the spinel type oxide. This proposal alleviates lattice mismatch between the MgO interlayer and the cobalt ferrite magnetic recording layer by the seed layer consisting of the oxide having the spinel structure which is analogous to the cobalt ferrite. However, this proposal does not teach or suggest in any way whether or not the seed layer consisting of the spinel type oxide is effective to magnetic recording layers consisting of a material other than the cobalt ferrite.
One of the factors which should be considered when intending improvement in the recording density of the magnetic recording layer is a distance between the magnetic recording layer and a magnetic head. In general, the recording density can increase, as the distance between the magnetic recording layer and a magnetic head decreases. On the other hand, the distance between the magnetic recording layer and a magnetic head must be larger than the surface roughness of the magnetic recording medium. Increase of the surface roughness of the magnetic recording medium leads to adverse influence to the flying characteristics of the magnetic head, so that the read/write characteristics of the magnetic recording medium may deteriorate. In the case of the interlayer consisting of conventionally used MgO, it is necessary to raise the substrate temperature during formation of the interlayer to not less than 400° C. for achieving the desired surface roughness. Therefore, there is a need for reducing the surface roughness of the magnetic recording medium, especially the surface roughness of the magnetic recording layer.